Display devices in which driving is performed using transistors including amorphous silicon or polysilicon are mainly used. However, it is difficult for these display devices to express multiple gray levels due to the influence of the off-state current of the transistors.
As an example of a pixel in a display device, FIG. 15 illustrates a pixel 5000 which includes a transistor 5001, a liquid crystal element 5002, and a capacitor 5003. The transistor 5001 includes amorphous silicon or polysilicon. In the pixel 5000, when image data is written to the liquid crystal element 5002 and the capacitor 5003 through the transistor 5001, an electric field is applied to the liquid crystal element 5002, so that images can be displayed.
However, due to the off-state current of the transistor 5001, electrical charges accumulated in the liquid crystal element 5002 and the capacitor 5003 are discharged, so that the voltage of the pixel fluctuates.
In the pixel 5000, the off-state current i of the transistor 5001, the storage capacitance C of the capacitor 5003, the fluctuation V in voltage, and the hold time T satisfy the relation of CV=iT. Therefore, if the off-state current i of the transistor 5001 is 0.1 pA (p indicates 10−12), the capacitance C of the capacitor 5003 is 0.1 pF, and one frame period is 16.6 ms, the fluctuation Vin voltage in the pixel in one frame period can be calculated as follows: 0.1 [pF]×V=0.1 [pA]×16.6 [ms]; thus, V=16.6 [mV].
If the display device has 256 (=28) gray levels and the highest drive voltage of the liquid crystal element in the pixel of 5 V, gray level voltage per gray level is about 20 mV. In other words, the fluctuation V (16.6 mV) in voltage in the pixel that is obtained from the calculation corresponds to the fluctuation in gray level voltage for about one gray level.
If the display device has 1024 (=210) gray levels, gray level voltage per gray level is about 5 mV. Therefore, the fluctuation V (16.6 mV) in voltage in the pixel corresponds to the fluctuation in gray level voltage for about four gray levels, and the influence of fluctuation in voltage due to off-state current cannot be ignored.
In Reference 1, a display device including a polysilicon transistor has been suggested.